1. Field of the Invention
The present invention relates to a harmonic wave generator in which a nonlinear optical member includes a resonator for resonating a fundamental wave so that the output harmonic wave can be modulated.
2. Discussion of Background
FIG. 5 is a diagram showing a conventional apparatus for producing a second harmonic wave in which an external resonator is used. In FIG. 5, reference numeral 1 designates a semiconductor laser (LD) for producing a fundamental wave, numeral 8 designates a collimate lens, numeral 3 designates a mirror used as an external resonator which resonates the fundamental wave (hereinafter, referred to as a .omega. light), numeral 8A designates a mode matching lens for matching a resonance mode in the external resonator with an incident beam, numeral 4 designates a nonlinear optical member such as a KNbO.sub.3 crystal or the like, numeral 5 designates a dichloic mirror which transmits the second harmonic wave (hereinbelow, referred to as a 2.omega. light) while the .omega. light is reflected, and numeral 2 designates a PZT element provided with a mirror for locking a resonant frequency by feeding-back the .omega. light to the semiconductor laser LD1. Numeral 6 designates a photodiode (PD) for detecting the .omega. light reflected by the dichloic mirror, and numeral 7 designates a controller which actuates the PZT element on the basis of a signal detected by the PD 6 whereby the angle of the mirror attached thereto is changed.
In the above-mentioned conventional second harmonic wave generator, it is important, in order to increase the luminous intensity of the second harmonic wave, that the phase of the .omega. light is matched with the phase of the 2.omega. light in the nonlinear optical member, namely, the refractive index n.omega. of the .omega. light is matched with the refractive index n2.omega. of the 2.omega. light in the nonlinear optical member.
In the conventional technique for such phase matching, the nonlinear optical member is constituted by a Peltier element or the like and the element is kept at a constant temperature so that the phase matching can be performed. Temperature control of the nonlinear optical member should be precise as .+-.0.1.degree. C. or less. The Peltier element stops the generation of the 2.omega. light due to a small change of temperature, and accordingly, the temperature should be precisely controlled so as to effect good phase matching.
There is another method to adjust the luminous intensity of the 2.omega. light by modulating the output of a semiconductor laser or the like which is used as a light source for the .omega. light. However, such method has a problem that the resonant frequency of the semiconductor laser by the .omega. light feed-back from the external resonator becomes easily unstable, whereby frequent generation of the 2.omega. light becomes impossible. Therefore, in the past, it was difficult to control precisely the luminous intensity of the 2.omega. light.
Further, the application of an electric field to the nonlinear optical member might cause twinning or breaking of the nonlinear optical member, and in the worst case, it may became in an inoperable state.